1) Field of the Invention
The present invention relates to a process for producing machinable ceramics containing fine crystals of fluorophlogopite dispersed in a vitreous matrix.
2) Description of the Related Art
Glass ceramics with fine crystals of mica evenly dispersed in a vitreous matrix have good machinability and excellent electrical and thermal properties are considered as promising materials for the enlargement of the application field of fine ceramics. In particular, glass ceramics containing fine particles of fluorophlogopite dispersed therein are useful materials also excellent in high-temperature stability.
Conventional processes for the production of fluorophlogopite-crystal-containing machinable ceramics include, for example, (1) processes in which glass obtained by fusing and then solidifying a mixture of raw materials is heated again to form crystals of fluorophlogopite in the glass, (2) processes in which a mass obtained by subjecting a gel of a precursor such as a metal alkoxide compound to heat treatment is ground, shaped and then heat-treated, whereby crystals of fluorophlogopite are formed, and (3) processes in which crystals of fluorophlogopite and a phosphate glass as a binder are mixed and are press-formed in a state heated in a suitable mold, followed by annealing.
In one example of the processes (1), a homogeneous glass batch is prepared using high-quality raw materials. The batch is fused at 1,300.degree.-1,450.degree. C. in a closed vessel, whereby a homogeneous melt is formed. This melt is then cooled into a glass body of a desired shape. The glass body is thereafter heated and maintained at 750.degree.-850.degree. C. so that the formation and growth nuclei of fluorophlogopite crystals are induced. The glass body is then heated and maintained at 850.degree.-1,100.degree. C., whereby the growth of crystals is allowed to proceed to completion (see Japanese Patent Application Laid-Open No. 2427/1972). Another example of the processes (1) is found in Japanese Patent Application Laid-Open No. 72654/1986 in which a mixture of pottery stones as principal raw materials, a fluorine compound, etc., are fused and formed into a glass body, followed by the crystallization of the glass body at 1,100.degree.-1,360.degree. C. in the presence of a volatile fluorine compound in a closed vessel.
As one example of the processes (2), an alkoxide compound and a non-alkoxide compound, the latter being soluble in a polar solvent, are mixed and dissolved or dispersed in a polar solvent and then heated, whereby the compounds are hydrolyzed into a gel-like state. The resultant gel is dried so that the polar solvent is caused to evaporate. The dry gel thus obtained is subjected to heat treatment at 800.degree.-1,100.degree. C. to drive off chemically-bonded organic components, whereby green crystals of fluorophlogopite are formed These green crystals are then into a green body. The green body is heat-treated 1,100.degree.-1,300.degree. C. so that fluorophlogopite crystals are allowed to grow (see Japanese Patent Application Laid-Open No. 178425/1986).
The processes (3) include the process disclosed in Japanese Patent Publication No. 21381/1970, in which a phosphate glass is mixed as a binder in fluorophlogopite crystals, the resultant mixture is heated to 500.degree.-700.degree. C. in a suitable mold and then press-formed there under a pressure of about 500-700 kg/cm.sup.2, and the thus-formed body is thereafter annealed at about 350.degree. C.
The production processes (1) in which the formation and growth of fluorophlogopite crystals are effected via a glass body individually comprise the following steps: fusion of raw materials, formation of a glass body from a melt, annealing, and crystallization. In the raw-material-fusing step, raw materials containing a fluorine component and 5-15%, on an oxide weight basis, of B.sub.2 O.sub.3 as a flux are fused at an elevated temperature of 1,300.degree.-1,450.degree. C. A vessel which is employed to fuse such raw materials must be made of a material (e.g., platinum) inert to the raw materials to avoid wearing. Means for minimizing evaporation of the raw materials, such as sealing means, is also indispensable.
In the glass-body-forming step, the above melt is poured into a desired mold at a temperature high enough to allow the melt to retain fluidity, and is then cooled to a temperature below the transition range thereof. Such a casting process however requires many molds for each desired shape upon mass production. The processes (1) therefore involves a problem in mass productivity.
The processes (1) are accompanied by further problems such that they need high thermal energy consumption; require a hermetic state to avoid evaporation of one or more components during the fusion or to maintain a fluorine-rich atmosphere upon crystallization; have difficulties in forming, for example, pipelike products because of the use of the casting technique of melt; may result, depending on the shape, in the occurence of a large machining loss when products of a desired shape are cut out since they are crystallized in the shape of the initial glass bodies; and the products have low heat resistance due to the abundant inclusion of B.sub.2 O.sub.3 on an oxide basis.
Each of the production processes (2) in which sintered products are obtained from alkoxide compounds and the like via gelation basically comprises the following steps: preparation of raw materials, gelation, first heat treatment, forming, and second heat treatment. Unless the moisture control of each raw material including a polar solvent is fully carried out during the preparation and gelation step for the raw materials, gelation takes place in the course of heating of the raw materials and an intended gel cannot be obtained. Accurate control of the reaction conditions is also necessary in the hydrolysis for the gelation in order to obtain a gel of stable quality. The first heat treatment step which is conducted at 800.degree.-1,100.degree. C. includes removal of chemically-bonded organic components contained in the dried gel, formation of fluorophlogopite crystals, and sintering to an appropriate degree. To achieve complete removal of the organic components, well-designed careful procedures are indispensable, including the need for a special method for filling the dried gel in a vessel and exposure to sufficient air (oxygen). Otherwise, the organic components tend to undergo carbonization and to remain in the heat-treated products, so that they may lead to impurities and/or pores in the final product. As a method for eliminating carbides by evaporation, it may be contemplated of exposing the final product to an oxidizing atmosphere of high temperature for a long time. This method however involves the potential problem that the fluorophlogopite crystals thus formed may be modified or the sintering may proceed too much. Even up to the step described above, difficulties are encountered upon setting conditions and conducting the steps, such as the control of gelation and the removal of organic components, so that products of stable quality can hardly be obtained. Further, for the second heat treatment, high temperature of at least 1,100.degree. C. is used to promote growth of fluorophlogopite crystals. Since fluorophlogopite crystals may undergo decomposition at such high temperature, it is necessary to control the heating by burying each green body in a fluorophlogopite powder called "packing powder" while sintering it in order to prevent decomposition.
Each of the production processes (3) in which fluorophlogopite crystals and a phosphate glass as a binder are heated, formed and annealed comprises the following steps: preparation of raw materials, heating and forming, and annealing.
In the raw material preparation step, glass powder and fluorophlogopite crystals are thoroughly mixed. Unless the mixing in this step is sufficient, the fluorophlogopite crystals may orient, thereby making it difficult or impossible to obtain a product with fluorophlogopite crystals evenly dispersed therein. This leads to a reduction in machinability. The production processes (3) are accompanied by another problem that extreme difficulties are encountered upon uniform dispersion and mixing of tabular or laminar fluorophlogopite crystals with glass powder. In the heating and forming step, the mixture is heated to 500.degree.-700.degree. C. and is then formed. The glass powder however has a composition of low melting point in view of the heat resistance of the material of the mold. Corollary to this, the heat resistance of the resulting product is considerably low.